Investigations have been launched on both sides of the Atlantic after a man was stopped at Heathrow Airport allegedly carrying bullets in his coat.
No flights were disrupted by the security scare
The 45-year-old Sudanese man, who flew to Heathrow from Washington DC's Dulles Airport on Wednesday morning, is being held in London under the Terrorism Act.
He was picked up by routine checks as he waited for a connection to Dubai.
The US Government says records from Dulles are being checked to see how he was able to board a flight there.
The investigation is being carried out by the Transport Security Administration (TSA), the US agency responsible for airport security.
A spokesman told the BBC the examination of airport records had so far shed no light on why the man was not picked up before boarding the Virgin Atlantic flight.
The man, who was initially arrested under the Firearms Act, was taken to a central London police station on Wednesday night to be interviewed by anti-terror detectives.
Police said he was not a known terrorist suspect.
The five bullets allegedly found by security staff are being examined by forensic experts.
Passengers who came in on the same flight said the man told staff he did not realise the items were in his coat pocket.
The British airline pilots' union, Balpa, has said "questions will be asked" about security at the Washington end.
Security at Dulles is particularly tight, with extensive checks performed on passengers and hand luggage before they are allowed anywhere near boarding areas.
A Virgin Atlantic spokeswoman confirmed the man had flown from Washington Dulles to Heathrow on flight VS022 in transit to board a flight to Dubai with another airline.
The airline said in a statement the "item" seized had not posed a threat to the aircraft.
"The safety and welfare of our passengers and crew is Virgin Atlantic's top priority," the statement said.
"Screening of passengers at Washington Dulles airport is the responsibility of the Transportation Security Administration."
Wednesday's incident follows American insistence that international flights to or over the US would be required to carry armed guards in certain cases over the festive period.
The decision came amid renewed fears that terrorists may be trying to use aircraft to stage a new attack.
The US request was followed by repeated disruption of the British Airways BA223 flight from Heathrow to Washington.
The flight was cancelled on 1 and 2 January and delayed on each of the next five days amid security fears and under guidance from the government.
A spokesman for British airline pilots' union Balpa said the detection of the man showed that security on the ground was crucial.
He said: "This is where we need to concentrate our investment, and not be diverted
into cul-de-sacs like sky marshals.
"Obviously there will be a lot of questions asked in America as to how he got on the plane undetected."
David Learmount, operations and security director at Flight International magazine, said security was ultimately dependent on human vigilance.
"You only need vigilance to lapse for one piece of luggage and that bag gets through", he said.